Intel Releases Critical Skylake And Kaby Lake HyperThreading Bug Fix

About a month ago we talked about a critical HyperThreading flaw that had been found in Intel Skylake and Kaby Lake processors. As we noted at that time, the flaw is very hard to trigger and is limited in scope. In fact, many people running processors affected by the flaw probably never noticed any stability issues with their machines as a result. With that said, no PC enthusiast wants a system that has any critical flaws lurking about, so Intel has fixed the issue with a new update.

The first indication that the flaw has been fixed comes from the Debian Linux user lists, the group that was initially credited with reporting the flaw. The message to the user list states that "Intel has issued public microcode updates in 2017-07-07, fixing the hyper-threading errata on every affected processor."

The updates to address the issue were included in the stable and oldstable point release of Debian issued on July 22nd. Updates to "intel-microcode" packages in the base version of 3.20170707.1 fix HyperThreading issues on every known flavor of Intel processor, including the Kaby Lake parts and all Skylake parts. The updated microcode is specific in not removing HyperThreading support as a workaround to the potential system instability issues seen with the original issue. The new microcode instead "fixes" HyperThreading

Debian User List states that updated microcode is related to the following errata:

Skylake D0/R0 (mobile/desktop), signatures 0x406e3, 0x506e3:
Known to be fixed in microcode revision 0xb9/0xba and later. Public fix available in linux microcode 20170511 and later.

Skylake H0 (server/HEDT/X-series), signature 0x50654:
Known to be fixed in microcode revision 0x2000022 and later, and it might have been fixed since revision 0x200001a. Public fix available in linux microcode 20170707 and later.

Kaby Lake H0/B0 (mobile/desktop), signatures 0x806e9, 0x906e9 (pf mask 0x22): Known to be fixed in microcode revision 0x5d/0x5e and later. Public fix available in linux microcode 20170707 and later.

Kaby Lake X-series, signature 0x906e9 (pf mask 0x08):
These processors are *NOT* affected when installed in a *supported* motherboard configuration (i.e. one that had its firmware updated to be compatible with Kaby Lake X-series). The launch production microcode already has the fix (believed to be microcode revision 0x5d or later based on the processor flags mask).

Kaby Lake Y0: signature 0x806ea:
Known to be fixed in microcode revision 0x65/0x66 and later, and it might have been fixed since revision 0x5d/0x5e. Public fix available in linux microcode 20170707 and later.

Microsoft issued a statement late last month noting it had released a software fix for Windows systems in April 2017 to address this flaw. When the HyperThreading issue first surfaced, the Debian Linux user group had recommended that users simply disable HyperThreading to avoid potential instability issues. With the fix now issued by Intel, any user who followed those recommendations can turn HyperThreading back on once the fix is applied. Again, be sure to download the latest BIOS updates from your motherboard or system manufacturer, to ensure you have this microcode fix.